As an exhaust gas-purifying catalyst that treats exhaust gas of an automobile, a three-way catalyst with precious metal supported by an inorganic oxide such as cerium oxide has been widely used. In the three-way catalyst, the precious metal plays the role in promoting the reduction of nitrogen oxides and the oxidations of carbon monoxide and hydrocarbons. Further, the inorganic oxide plays the roles in increasing the specific surface area of the precious metal and suppressing the sintering of the precious metal by dissipating heat generated by the reactions. In particular, cerium oxide has an oxygen storage capacity and is capable of optimizing the oxidation and reduction reactions.
However, when the three-way catalyst is used under high temperature conditions, the crystal grains of cerium oxide grow in size. As a result, the surface area of cerium oxide decreases. Decrease in the surface area of cerium oxide causes decrease in oxygen storage capacity. For this reason, various researches and developments have been conducted in order to improve heat stability of cerium oxide.
For example, JP-B 6-75675 describes an exhaust gas-purifying catalyst that contains a composite oxide of cerium and zirconium. JP-A 2000-169148 describes that a composite oxide of cerium, zirconium and yttrium is used. JP-A 11-21171 describes that an exhaust gas emitted by an internal combustion engine is purified using an oxygen storage material made of a composite oxide of cerium and at least one of praseodymium, lanthanum, yttrium and neodymium.
These composite oxides are superior to cerium oxide in terms of heat stability. Therefore, these composite oxides can deliver excellent oxygen storage capacity for a long period of time.
However, the present inventors have found the following facts in the course of achieving the present invention. When three-way catalysts using the above composite oxides are used for a long period of time under high temperature conditions, for example, at 1,000° C. or higher, growth of precious metal particles occurs, though growth of the composite oxide particles is less prone to occur. In addition, the oxygen storage capacities of the composite oxides decrease as the precious metal particles grow.